MEASUREMENT OF THE ATMOSPHERIC NEUTRINO-INDUCED UPGOING MUON FLUX USING MACRO

We present a measurement of the flux of neutrino-induced upgoing muons (< E-nu > similar to 100 GeV) using the MACRO detector. The ratio of the number of observed to expected events integrated over all zenith a ngles is 0.74 +/- 0.036 (stat) +/- 0.046 (systematic) +/- 0.13 (theore tical). The observed zenith distribution for - 1.0 less than or equal to cos theta less than or equal to -0.1 does not fit well with the no oscillation expectation, giving a maximum probability for chi(2) Of 0. 1%, The acceptance of the detector has been extensively studied using downgoing muons, independent analyses and Monte Carlo simulations. The other systematic uncertainties cannot be the source of the discrepanc ies between the data and expectations. We have investigated whether th e observed number of events and the shape of the zenith distribution c an be explained by a neutrino oscillation hypothesis. Fitting either t he flux or zenith distribution independently yields mixing parameters of sin(2)2 theta = 1.0 and Delta m(2) of a few times 10(-3) eV(2). How ever, the observed zenith distribution does not fit well with any expe ctations, giving a maximum probability for chi(2) of 5% for the beat o scillation hypothesis, and the combined probability for the shape and number of events is 17%. We conclude that these data favor a neutrino oscillation hypothesis, but with unexplained structure in the zenith d istribution not easily explained by either the statistics or systemati cs of the experiment. (C) 1998 Published by Elsevier Science B.V. All rights reserved.